Field of the Invention
The present invention relates to a threshold voltage sensing circuit of an organic light-emitting diode (OLED) display device, and more particularly, to a threshold voltage sensing circuit of an OLED display device, which changes the threshold voltage of an OLED to a voltage suitable for protecting a low-voltage driving element in an analog-to-digital converter, when sensing the threshold voltage of the OLED and outputting the sensed threshold voltage to the analog-to-digital converter.
Description of the Related Art
In general, a display panel of an OLED display device includes a plurality of pixels arranged in a matrix shape, and each of the pixels includes an OLED. When a signal is supplied to a gate line, each of the pixels is turned on by a data signal supplied from a data line, and emits light. The unit pixels of the display panel include OLEDs arranged therein and showing a unique color of red, green, and blue. The colors of the OLEDs may be combined to express a target color.
However, since the OLEDs on the display panel gradually deteriorate with time, the threshold values thereof are changed. Thus, although the same driving current is supplied to the OLEDs, the brightness of the OLEDs may be gradually changed with time.
Thus, the threshold voltages of the OLEDs may be sensed and stored in a memory. When a data signal is outputted to the display panel, the data signal may be compensated for according the changes of the threshold voltages based on the stored threshold voltages. Therefore, the OLEDs may maintain constant brightness at all times, regardless of the use time of the OLEDs.
FIG. 1 is a block diagram of a conventional threshold voltage sensing device of an OLED display device. As illustrated in FIG. 1, the conventional threshold voltage sensing device includes a display panel 10, a gate driver 20, a source driver 30, and a threshold voltage sensing controller 40.
Each of pixels arranged in the display panel 10 includes a switching transistor TFT-S which transmits a data signal to a driving transistor TFT-D through data lines DL1 to DLn of the source driver 30. The driving transistor TFT-D supplies a driving current corresponding to the data signal supplied through the switching transistor TFT-S to the corresponding OLED. A capacitor C coupled between one terminal and the gate of the driving transistor TFT-D and maintains the turn-on state of the driving transistor TFT-D during one frame, the corresponding OLED may maintain the light-emitting state during one frame.
Before the system is powered on to display an image on the display panel 10 or in a threshold voltage sensing mode, the threshold voltage sensing controller 40 sequentially outputs a control signal to threshold voltage compensation control lines CL1 to CLn. Thus, threshold voltage sensing transistors TFT-V of a corresponding horizontal line are sequentially turned on.
When the control signal is supplied to the first threshold voltage compensation control line CL1 to turn on the threshold voltage sensing transistors TFT-V, the source driver 30 transmits precharge voltages to the data lines DL1 to DLn through buffers BUF1 to BUFn, respectively. At this time, the precharge voltages are supplied to the anodes of the OLEDs, respectively.
Then, when the precharge voltages of the OLEDs are sufficiently discharged, sample and hold circuits SH1 to SHn sample and hold the threshold voltages Vth of the OLEDs, sensed through the threshold voltage sensing transistors TFT-V and the corresponding data lines DL, respectively. The analog threshold voltages Vth sampled and held through the sample and hold circuits SH1 to SHn are converted into digital signals through an analog-to-digital converter 31, and stored in a memory.
Subsequently, the same operation is repeated on the next horizontal line. Whenever the same operation is repeated on each horizontal line, the threshold voltages of the OLEDs are converted into digital signals and stored in the memory.
In an image display mode, when data signals are outputted to the OLEDs, the data signals may be compensated for as much as the changes of the threshold voltages based on the threshold voltages stored in the memory. Thus, the OLEDs maintain the constant brightness regardless of the changes of the threshold voltages.
However, since the sample and hold circuits SH1 to SHn and the analog-to-digital converter 31 perform a digital logical circuit operation, the sample and hold circuits SH1 to SHn and the analog-to-digital converter 31 are typically implemented with transistors which are driven at a low voltage. Thus, when a threshold voltage is sensed and transmitted to the analog-to-digital converter 31, the PN-junction diode of the transistor (for example, LV PMOS transistor) may be turned on in case where the threshold voltage is higher than the limit voltage (for example, VDD+Vth) which guarantees stable operations of the transistors within the analog-to-digital converter 31. Thus, a discharge operation may occur due to leakage current in the analog-to-digital converter 31.
Nevertheless, the conventional threshold voltage sensing device does not include a function of changing or limiting a sampled and held threshold voltage to the limit voltage or less, which guarantees the stable operations of the transistors within the analog-to-digital converter. Thus, a discharge operation may be caused by leakage current, and the values of the threshold voltages sensed from the OLEDs may not be normally stored in the memory.